Speaker 1: Hey, are you welcome to Stuff to Blow Your Mind?
Speaker 1: My name is Robert Lamb and I'm Joe McCormick, and
Speaker 1: we're running some vault episodes because we're out this week.
Speaker 1: This episode originally aired February third, twenty twenty two, and
Speaker 1: it's part two of our series on thirst. Get right
Speaker 1: in Welcome to Stuff to Blow Your Mind, a production
Speaker 1: of iHeartRadio. Hey, welcome to Stuff to Blow your Mind.
Speaker 1: My name is Robert Lamb and I'm Joe McCormick, and
Speaker 1: we're back with part two of our series on thirst.
Speaker 1: What are you drinking right now? Absolutely nothing? Oh no, wait,
Speaker 1: I do have I have water here at my desk. Water.
Speaker 1: I'm always drinking water. I'm a water guy. You know,
Speaker 1: some people are really into tea. I almost never drink tea.
Speaker 1: I go coffee water. That That's pretty much it during
Speaker 1: the daytime hours at least. Oh yeah, well, I'm currently
Speaker 1: drinking tea. I generally go coffee tea, tea, water tea,
Speaker 1: and then water, and then when the evening comes, you know,
Speaker 1: maybe I'll have something something else to drink. But yeah,
Speaker 1: right now it is tea, which is essentially water but
Speaker 1: with extra things added to it. You know, I sometimes
Speaker 1: get confused about what tea means because I think of
Speaker 1: tea as being a specific kind of plant, Like, isn't
Speaker 1: there a tea tree? Again, I don't know much about tea,
Speaker 1: so I think about it in that way. But then
Speaker 1: you can make a tea out of like anything basically
Speaker 1: that you steep in water, right, people make mushroom teas,
Speaker 1: people with all kinds of just like oh yeahs and
Speaker 1: plant stuff in water, and that's tea. Also, yeah, like
Speaker 1: I mean, there's there's proper tea, and that's generally what
Speaker 1: I'm what I'm drinking. I'm drinking, you know, a dark
Speaker 1: tea or maybe a green tea. But also sometimes I'll
Speaker 1: have a tumeric tea, which is like like tumeric and
Speaker 1: ginger and a little coconut oil and a little honey
Speaker 1: hot water. But it doesn't actually contain tea, So it's
Speaker 1: not completely accurate to call it such, because it's sort
Speaker 1: of how not all band aids are technically band aids
Speaker 1: or xeroxis or whatever. Has this been a terrible intro,
Speaker 1: Maybe we can cut my tea thing. No, No, this
Speaker 1: is a This is an intro that the people can
Speaker 1: relate to people are can have thoughts on this. It's
Speaker 1: always good to start off asking what a word means.
Speaker 1: So I turns out I know nothing about tea. I
Speaker 1: have no no, there you go. We need to come
Speaker 1: back and do an episode or a series of episodes
Speaker 1: on t It's a fascinating subject, fascinating history. Okay, well,
Speaker 1: we're going to jump back into some of the science
Speaker 1: that we were talking about in the last episode, where
Speaker 1: I was reviewing an interesting paper that I had read
Speaker 1: summarizing the recent state of research on the biology of thirst.
Speaker 1: That was it was a paper published in Current Biology.
Speaker 1: We had to break off in the middle of talking
Speaker 1: about it. But but before we get back into that, Rob,
Speaker 1: I think you want to talk about the taste of water,
Speaker 1: which I think is in itself a very interesting subject. Yeah,
Speaker 1: it's it's a strange one because we probably don't think
Speaker 1: about it enough, but but we often talk about it,
Speaker 1: especially when we when we travel from one place to another.
Speaker 1: UM we if you look back on I think back
Speaker 1: on water that you've consumed in the past, you may
Speaker 1: have specific memories of different waters. Um Like I remember
Speaker 1: when my family lived in the country and we had
Speaker 1: well water and it had it had a distinctive taste,
Speaker 1: and I don't I can't say that I loved it,
Speaker 1: But if I were to encounter the exact same flavor
Speaker 1: profile again, it would probably make me nostalgic, you know,
Speaker 1: like it it's it's a definite flavor that is tied
Speaker 1: to a definite place and time. Warning that this is
Speaker 1: a little crude, but I don't know of how else
Speaker 1: to explain it. You've ever been somewhere that had farty water, Um, well,
Speaker 1: it's kind of sulfury water. Yeah, it kind of kind
Speaker 1: of like water from the Yes, that's kind of what
Speaker 1: this was like. Yeah, it was that had kind of
Speaker 1: a fire and brimstone kind of flavor to it. Oh okay,
Speaker 1: that's a more elegant way to put it. Well, the
Speaker 1: denizens of Hell are not insalted. They're flatter that you
Speaker 1: would you would compare this foul drinking water two flats,
Speaker 1: uh huh. But um, you know, but then sometimes I
Speaker 1: don't know, there's there's also beech water, you know, like
Speaker 1: I anytime I'm in Florida and I'm having Florida water,
Speaker 1: like it has its own it tends to have its
Speaker 1: own bouquet, its own flavor. Profile that at the time
Speaker 1: I'm not enjoying, but then after I'm back and drinking
Speaker 1: taste your water, I'm kind of kind of long for it,
Speaker 1: you know, because like this is the water of the
Speaker 1: vacation that I am no longer on that sort of thing.
Speaker 1: I remember when I was in like a middle school,
Speaker 1: I went to New Mexico and we went somewhere there
Speaker 1: where the water was very I don't know, the tap
Speaker 1: water was almost kind of frothy somehow. It was like
Speaker 1: a kind of kind of white and cloudy, and I
Speaker 1: remember thinking it had a strange taste. I don't know
Speaker 1: what to connect that too, if it was supposed to
Speaker 1: be like that or not. Yeah, So I guess one
Speaker 1: thing we can definitely establish here is that there certainly
Speaker 1: seems to there is an objective difference in drinking waters
Speaker 1: from one place to another, Atlanta water versus Florida water,
Speaker 1: that sort of thing. But there's also this wide variety
Speaker 1: in how people just interpret the taste of water, you know.
Speaker 1: I think we often think of water as being neutral
Speaker 1: or tasteless. If you're mixing a cocktail, for example, you
Speaker 1: don't want to shake it too long with the crushed
Speaker 1: ice because you will what will happen, You will water
Speaker 1: it down. Water in this case is the antithesis of
Speaker 1: an interesting flavor profile. Well, but the other half of
Speaker 1: that is sometimes people screw up making a drink because
Speaker 1: they don't shake it with ice at all when they're
Speaker 1: supposed to, and they don't understand that shaking with ice
Speaker 1: not only cools the drink, but it also adds a
Speaker 1: certain amount of water to the drink, which is an
Speaker 1: important ingredient. Yes, absolutely so. You suppose to shake a
Speaker 1: cocktail and you don't, you end up with a drink
Speaker 1: that's usually too strong or too sweet. Yeah, there's a
Speaker 1: balance to be maintained there, much like the balance of
Speaker 1: water in the human body that we discussed in the
Speaker 1: last episode. Many of you out there have probably met
Speaker 1: someone who claims to not like the taste of water.
Speaker 1: Sometimes these individuals will will use flavored water or other
Speaker 1: beverages instead, and we'll tend to shy away from just
Speaker 1: drinking straight water. But I don't know if you're like me,
Speaker 1: you know, perhaps you've noticed times when a glass of
Speaker 1: ice water is just super satisfying. Um, you know, particularly
Speaker 1: on a hot day, like a just a super cold
Speaker 1: water lots of ice in it. Um I would, I
Speaker 1: would tend to. I found myself wanting to categorize that
Speaker 1: as delicious, though at the same time feeling weird for
Speaker 1: thinking that because it's like it's water. I can't say
Speaker 1: it's delicious. It has no flavor, right, that's what we believe.
Speaker 1: Another another one I really like is the first sip
Speaker 1: of cold water that I have after I have brushed
Speaker 1: my teeth. Now, after I've brushed my teeth and rinsed
Speaker 1: and spit I'm not drinking water that way, but yeah, like,
Speaker 1: after I've brushed my teeth, I've walked into another room,
Speaker 1: I have a sip of water, super cool and refreshing. Agreed,
Speaker 1: much better than the sip of orange juice after you've
Speaker 1: brushed your teeth, which is, Yeah, I don't know if
Speaker 1: you've ever tried that, it's famously disgusting. Oh this is
Speaker 1: this is always the worst. Really getting into the weeds here,
Speaker 1: but that that feeling when you you were you're busy
Speaker 1: in the morning, you go brush your teeth, you come
Speaker 1: back and you find your coffee cup and it still
Speaker 1: has half a cup of coffee in it. Oh, heartbreaking,
Speaker 1: because you know you can't drink it. Now. If you drink,
Speaker 1: you can drink it, you can you can throw it back,
Speaker 1: but it's going to taste awful. I wonder if anybody's
Speaker 1: ever done a controlled study of how long you have
Speaker 1: to wait after brushing your teeth before those those horrible
Speaker 1: flavor interactions fade away. I don't think it's too long,
Speaker 1: it's yeah. I mean, if I had to venture to guess,
Speaker 1: I'd say it couldn't be more than like twenty minutes, right, Yeah,
Speaker 1: of course, a lot of this, some of this is
Speaker 1: going to vary culture to culture. In some cultures, it's
Speaker 1: not it's not considered advisable to drink cold water. You
Speaker 1: want to drink hot water. So you know, there's it's
Speaker 1: gonna it's gonna it's gonna vary. There's a lot of
Speaker 1: cultural stuff going on here as well. Oh I've never
Speaker 1: heard of that. What's what's an example of a culture
Speaker 1: that favors hot water drinking. There's a connection if memory
Speaker 1: serves as a connection to traditional Chinese medicine here, where
Speaker 1: the idea is that it's better for your health to
Speaker 1: drink hot water as opposed to cold water. But I
Speaker 1: don't remember any of the deeper details of regarding it all. Right, Well,
Speaker 1: to get us back on track, you had mentioned that
Speaker 1: maybe maybe it was wrong to say that tap water,
Speaker 1: you know, cold water coming in on a hot day
Speaker 1: was delicious because water itself doesn't really have a taste.
Speaker 1: And I think this is a common understanding. But is
Speaker 1: that true? Like it does it have a taste? Does
Speaker 1: it not have a taste? Does it have a taste?
Speaker 1: We're just so used to that we can't taste it anymore. Yeah,
Speaker 1: we'll like to. Well, to go back to Aristotle, Aristotle
Speaker 1: was certainly kind of in the no taste camp, stating
Speaker 1: that it was for the most part tasteless. The idea
Speaker 1: that water is that it made liver flavors, but it's
Speaker 1: in and of itself does not have flavor. But a
Speaker 1: lot of a lot of work has gone into this
Speaker 1: this question. Uh, some some interesting studies. UM. I was
Speaker 1: reading an article by Berlin Game at All titled Understanding
Speaker 1: the Basics of Tapwater Taste, publishing the American Water Works
Speaker 1: Association Journal in two thousand and seven. UM, and they
Speaker 1: point out that the basically you get the water is
Speaker 1: going to contain negatively and positively charged ions as minerals,
Speaker 1: you're going to find that in your tap water, and
Speaker 1: that can positively and negatively affect taste. This is especially key,
Speaker 1: they point out as far as water regulations and reverse
Speaker 1: osmoses go, which is to say, without without getting into
Speaker 1: all the chemistry, there are certain differences in water flavor
Speaker 1: due to different minerals and other elements in the water. Now,
Speaker 1: another interesting thing about about tap water, especially generally when
Speaker 1: you're drinking tap water, the drinking water probably contains calcium, magnesium, sodium,
Speaker 1: and according to Azolae at All in an article in
Speaker 1: the Journal of General Internal Medicine from two thousand and
Speaker 1: one titled Comparison of Mineral Content of tap water and
Speaker 1: bottled waters, the mineral content of drinking water like this
Speaker 1: ultimately may be an important source of daily recommended mineral dosages,
Speaker 1: certainly in the United States, which was the area they
Speaker 1: were looking at here. And so basically, while we might
Speaker 1: think of water as being this thing that we would
Speaker 1: will just purify the heck out of it, just give
Speaker 1: me the purest water possible, the purest water possible isn't
Speaker 1: necessarily going to be the healthiest or the tastiest, which
Speaker 1: I thought was an interest distinction, like when when we
Speaker 1: when we purchase bottled water, we're not going and buying
Speaker 1: the distilled water usually. I mean, maybe, you know, maybe
Speaker 1: that's what you are doing, but for the most part,
Speaker 1: people are are not buying distilled water for drinking purposes. Right.
Speaker 1: But distilled water, even though it is probably the purest
Speaker 1: form of water you can get, is not necessarily better
Speaker 1: for any reason, not for not for health or for
Speaker 1: experience of drinking. Right now, in terms of just trying
Speaker 1: to get down to the question though, well, what what
Speaker 1: does it taste like? Okay, you know, we're saying it
Speaker 1: can can go a little in this direction, a little
Speaker 1: in that direction. We have these mineral components, uh, you know,
Speaker 1: adding to the to the flavor. But but what is
Speaker 1: the taste itself? And I think one of the interesting
Speaker 1: things about that question is that it forces you to
Speaker 1: take a step back and think about taste itself. Um,
Speaker 1: you know, taste involves both sensations on the tongue and
Speaker 1: olfactory information as well. So it's not only it's not
Speaker 1: only tasting with the mouth, it's smelling, uh whatever you
Speaker 1: know it is that you or consuming as well. And
Speaker 1: certainly we do taste contents in the water, though under
Speaker 1: ideal circumstances that that's not going to push you hard
Speaker 1: in any direction. But obviously, if you dilute some sugar
Speaker 1: in a glass of the universal solvent and drink it,
Speaker 1: you'll find it sweet. Saltwater taste salty. Your sense of
Speaker 1: taste is still weighing in on the water. But in
Speaker 1: terms of this kind of brings us back to the
Speaker 1: same question, does the water itself have a taste or
Speaker 1: is it just the vehicle for these various flavors, be
Speaker 1: it salt or sugar, or a slight hint of magnesium,
Speaker 1: that sort of thing. So one idea proposed in the
Speaker 1: sixties and seventies by psychologist Linda Bartschuk was that the
Speaker 1: taste of water is more of an after taste of
Speaker 1: prior eating and drinking. And this, too, I guess, is
Speaker 1: more in line with the idea of water as the
Speaker 1: vehicle of flavor, but something that is in and of
Speaker 1: itself flavorless. So the idea would be that, like when
Speaker 1: you take a drink of water, you are somehow re
Speaker 1: experiencing flavors of foods you may have eaten most recently. Yeah,
Speaker 1: and this this will become important. And again we have
Speaker 1: to realize that when you take a sip of water,
Speaker 1: you are not introducing water. No matter how sterile your
Speaker 1: water is, your mouth is not sterile. You know your
Speaker 1: your mouth is going to even if you if you've
Speaker 1: just brushed your teeth, great, but that you're going to
Speaker 1: have some semblance of the of that experience in your mouth.
Speaker 1: If you're eating dinner, you know there's going to be
Speaker 1: the hint of food or drink as well. So, yeah,
Speaker 1: you're not introducing the water into a neutral place. If
Speaker 1: nothing else you your saliva is present. Emily Underwood wrote
Speaker 1: in an excellent short twenty seventeen piece for the American
Speaker 1: Association for the Advancement of Science on this topic, the
Speaker 1: Flavor of Water, And at one point she's a sighting
Speaker 1: Zachary Knight, Oh, who is one of the three authors
Speaker 1: of the research summary thirst in Current Biology that I
Speaker 1: was referring to in the last episode and I'll be
Speaker 1: talking about again in a few minutes. Yeah, So Underwood
Speaker 1: points to something that that we discussed in the last
Speaker 1: episode that you know, ultimately, when we're getting into taste,
Speaker 1: when we're getting into water detection in the mouth, the
Speaker 1: molecular and cellular mechanisms here are not that well understood.
Speaker 1: And then she also touches on some of nights research
Speaker 1: regarding the thirst trigger. Uh, you know where where that
Speaker 1: is seems to locate in the brain and then ultimately
Speaker 1: what's where is the trigger in the in the rest
Speaker 1: of the body. But we'll come back to that in
Speaker 1: a bit. Yeah, but I guess there are somewhat separate
Speaker 1: questions for can you detect water in your mouth versus
Speaker 1: can you taste water? I mean those are slightly slightly different, right, yeah, well,
Speaker 1: but but then again when you start thinking about like
Speaker 1: what taste is, maybe they're not that different, like it's
Speaker 1: but but it basically comes down to, yeah, it's not
Speaker 1: about interpreting these minerals or sweetness or saltiness, but like
Speaker 1: the just the basic signal of it is water. It
Speaker 1: is in my mouth, you know, and then ultimately being
Speaker 1: able to tell if something is not water, like or
Speaker 1: if it or if the water is you know, too
Speaker 1: far in a particular direction, like oh it is it
Speaker 1: is actually oil and water and not just water. I
Speaker 1: wanted water. That's sort of question. So research has been
Speaker 1: trying to isolate water sensing taste receptor. It sells on
Speaker 1: the tongue of a particular note. Here is the paper
Speaker 1: The Cellular Mechanism for Water Detection in the Mammalian Taste
Speaker 1: system by Zocchi at All, published in Nature Neuroscience in
Speaker 1: twenty seventeen. This was a team i believe from the
Speaker 1: California Institute of Technology in Pasadena, and they were working
Speaker 1: with mice and this is this is one of those
Speaker 1: the studies that I mean, it's already an interesting area,
Speaker 1: like using mice to figure out how we're tasting, but
Speaker 1: and it goes in ultimately strange directions that are pretty fascinating.
Speaker 1: So they were they were using different varieties of mice
Speaker 1: with specific types of taste receptor cells genetically knocked out
Speaker 1: in order to try and isolate which ones were seemingly
Speaker 1: useful in tasting water. And they found that the acid
Speaker 1: sensing sour taste receptor cells seem to be the most involved.
Speaker 1: Mice with those knocked out took far longer distinguishing oily
Speaker 1: water from clean water. In the words of the study,
Speaker 1: this quote compromise discrimination between water and non aqueous fluids.
Speaker 1: So these cells seem like they may they may well
Speaker 1: be very much involved in the process. Interesting, Okay, so
Speaker 1: if we are actually tasting water, it may rely more
Speaker 1: on the cells that normally taste sourness or acidity than
Speaker 1: other taste receptor cells, or at least in mice, And
Speaker 1: if the same held true in humans, that would be
Speaker 1: the case for us. Right. But this is where the
Speaker 1: mouse experiment from this team gets weird. So to further
Speaker 1: test this out, they bred mice that could taste light
Speaker 1: with these acid sensing TRCs taste receptor cells. They train
Speaker 1: them to drink water from a spigot, and they replace
Speaker 1: that spigot with an optic fiber cable. So they apparently
Speaker 1: treated the mice in this case, they apparently treated the
Speaker 1: light as if they were tasting water, but they didn't stop.
Speaker 1: They kept drinking the light long after they would have
Speaker 1: normally stopped drinking water. So the acid sensing TRCs might
Speaker 1: be involved in triggering drinking, but they might not be
Speaker 1: involved in stopping you from drinking, like saying, Okay, well,
Speaker 1: that's that's all the water I need to drink right now,
Speaker 1: and I'm not sure there may be other complications there.
Speaker 1: I mean, once you have an animal drinking light and
Speaker 1: not actual water. It just seems to me like there
Speaker 1: might there might be other things going on there. I'll
Speaker 1: on a very physical basis, so this would be taste
Speaker 1: receptor cells in the mouth, but they're like optically sensitive cells.
Speaker 1: Am I right about this? It's literally like drinking light
Speaker 1: with their mouth, like letting light shine into the oral cavity. Yes,
Speaker 1: it h I mean, it sounds insane when you say
Speaker 1: it out loud, but but yeah, there's there's actually a
Speaker 1: YouTube video about it from Science Magazine titled these mice
Speaker 1: are drinking light. So if anyone needs to actually see
Speaker 1: what we're talking about here, pull that up. And yeah,
Speaker 1: just it's it's a this, this black mouse going up
Speaker 1: to this um this little receptacle, and it appears to
Speaker 1: be drinking, but there's blue light flooding out of the hole, okay,
Speaker 1: and it's got light sensitive cells in its mouth, So
Speaker 1: the mouse's brain is reacting as if by shining light
Speaker 1: into its mouth it was swallowing water. Yes, that's crazy, man,
Speaker 1: Yeah it is. This is this is it's it's mind
Speaker 1: blowing on several levels here. So ultimately this this all
Speaker 1: might be related to changes in pH level when in
Speaker 1: a normal situation, when you know, a mouse or any
Speaker 1: mammal supposedly is drinking something that is not light. When
Speaker 1: it tries out the water, saliva is washing around in
Speaker 1: the mouth, and the removal of saliva might be key
Speaker 1: to sensing water. Again, coming back to what we said
Speaker 1: earlier about you know, water is not entering into a
Speaker 1: neutral environment when it goes in your mouth. It's going
Speaker 1: to interact with at least saliva, and in doing so
Speaker 1: that might change the pH level which triggers these these
Speaker 1: TRCs on the tongue. That seems to be the basic
Speaker 1: framework we're potentially looking at here. Oh, that's interesting. So
Speaker 1: at least according to the idea here, part of the
Speaker 1: sensation of drinking water might be the water stability to
Speaker 1: wash natural saliva out of the mouth. Right though, of
Speaker 1: course standard caveat that. More researches quired and then we
Speaker 1: still don't We still don't have this one knocked one,
Speaker 1: but it's yeah, it's it. Ultimately, it makes you rethink
Speaker 1: again what taste actually is and consider that there may
Speaker 1: be precise triggers in place for detecting water and distinguish
Speaker 1: it from other liquids. And then also letting you know
Speaker 1: that you have had enough water. Like all these things
Speaker 1: that we just take for granted, we think of them
Speaker 1: as we think of them as choices that we make,
Speaker 1: like I have I am, I've decided now I must
Speaker 1: have water. I have decided now that I don't need
Speaker 1: any more water. But but these are all tied into
Speaker 1: the you know, this intricate biological system. Than all right, well, rob,
Speaker 1: if you're ready, I wanted to discuss some more of
Speaker 1: the things I was reading in that twenty sixteen article
Speaker 1: in Current Biology that was about the biology of thirst. Again.
Speaker 1: This was by David Leeb, Christopher Zimmerman, and Zachary Knight.
Speaker 1: Let's do it all right now. We already talked about
Speaker 1: some of the conditions that will trigger thirst and water
Speaker 1: seeking behavior in animals such as rodents. In humans, commonly,
Speaker 1: one thing would be a decrease in water volume in
Speaker 1: the body. But another thing would be an increase in
Speaker 1: what's called blood osmolality osmol A L I T Y
Speaker 1: I spell it because that word will keep coming up
Speaker 1: in this discussion. Osmolality is the concentration of substances dissolved
Speaker 1: in the water content of the body. So the blood
Speaker 1: osmolality will go up, say if you consume salt or
Speaker 1: consume other kinds of salts of various minerals, whether that's
Speaker 1: sodium or magnesium or whatever, there are lots of things
Speaker 1: dissolved in the blood, and as the concentration of those
Speaker 1: things dissolved in the blood goes up, that's known as
Speaker 1: increasing osmolality. And as we discussed in the last episode,
Speaker 1: you your osmolality really needs to be pretty tightly constrained
Speaker 1: within an ideal range, or it can start leading to
Speaker 1: system wide problems with cellular function because cells need a
Speaker 1: pretty tightly controlled electrochemical gradient on each side of their
Speaker 1: membranes in order to control the passage of ions in
Speaker 1: and out of the cell to you know, take in things,
Speaker 1: the cell needs to release waste products and so forth.
Speaker 1: So in order for your body to work right, it
Speaker 1: needs to have the right level of the right concentration
Speaker 1: of things like salts dissolved in its water. But you
Speaker 1: also have to keep your body, your body's water volume
Speaker 1: at the right level in order to maintain ideal blood
Speaker 1: pressure because that blood's got to flow, and if you
Speaker 1: suddenly are to remove a lot of liquid from the body,
Speaker 1: suddenly the heart has to pump harder and harder to
Speaker 1: get the red blood cells to all the different parts
Speaker 1: of your body. So keeping up keeping up the right
Speaker 1: amount of water in the body and the correct concentration
Speaker 1: of substances dissolved in that water is crucial. And that
Speaker 1: dichotomy we talked about last time actually breaks down into
Speaker 1: two distinct types of dehydration that the authors talk about,
Speaker 1: and these two types of dehydration actually lead to different
Speaker 1: behavioral reactions in animals. So you can have dehydration within
Speaker 1: cells or dehydration between cells. A loss of water from
Speaker 1: within cells, known as intracellular dehydration, is usually caused by
Speaker 1: high blood osmolality, so the introduction of salts or other
Speaker 1: things like that into the body. This draws water out
Speaker 1: of cells by osmosis and causes the cells to shrink,
Speaker 1: which certainly doesn't sound good. Standard behavioral response to that
Speaker 1: kind of thing that intracellular dehydration is thirst. You want water,
Speaker 1: so you go get it. Loss of water from between
Speaker 1: cells is known as extra cellular dehydration, and this usually
Speaker 1: is caused by a loss of total blood volume, for
Speaker 1: example by bleeding. You know, if you cut yourself and
Speaker 1: lose a bunch of liquid out of your arm or something.
Speaker 1: You will lose total blood content without changing the body's osmolality.
Speaker 1: You know, you think about it that way, like you're
Speaker 1: the liquid is going down, but you're not changing how
Speaker 1: salty the liquid that's left is. So in order to
Speaker 1: recover from that condition, you will actually need both water
Speaker 1: and salt to replenish the lost volume. Just drinking water
Speaker 1: alone would leave your osmolality too low. So the behavioral
Speaker 1: response to loss of total water content or content from
Speaker 1: between cells is usually thirst plus what the author is
Speaker 1: called salt appetite. You want water and salt at the
Speaker 1: same time, But interestingly, many things that happen to the
Speaker 1: body cause both types of dehydration at the same time.
Speaker 1: Then the example they give is sweating. This is very common, right,
Speaker 1: you go out in the sun and you sweat. Well,
Speaker 1: sweat is not only a loss of blood volume. Sweat
Speaker 1: is salty, but sweat is actually less salty than your blood.
Speaker 1: So if your body is losing liquid that is less
Speaker 1: salty than its water content overall, like sweat is, the
Speaker 1: salt content of what's left behind is therefore increasing. Does
Speaker 1: that make sense, So the extra salt left over inside
Speaker 1: you when you sweat causes an increase in blood osmolality,
Speaker 1: again triggering a thirst for water. It's kind of counterintuitive
Speaker 1: because if you ever do taste sweat it, you know,
Speaker 1: it tastes salty, so you would think it would feel
Speaker 1: like you're losing salt, but you're actually gaining salt is
Speaker 1: in relationship to the amount of water left in you. Yeah,
Speaker 1: I think we've touched on this on the show before
Speaker 1: that sometimes we give too much credit to sweat in
Speaker 1: its ability to remove things from our body. Certainly when
Speaker 1: some arguments for the removal of impurities through sweat, for example,
Speaker 1: Oh yeah, yeah, right with people who think that you
Speaker 1: can like cleanse all the toxins by sweating or something.
Speaker 1: I mean, sweating is great, no, no knocking on sweat
Speaker 1: and it memory serves like there is some toxin removal,
Speaker 1: but not not anywhere near as good as say, good
Speaker 1: old fashioned urination. I mean, that's that's right, That's why
Speaker 1: we have urination exactly. So I guess from here we
Speaker 1: go on to talk about how the brain senses and
Speaker 1: monitors osmolality. This is what the authors say is quote,
Speaker 1: probably the most important homeostatic signal for drinking in everyday
Speaker 1: life is the brain's ability to monitor osmolality. That's what's
Speaker 1: going to be causing you to go drink water. And
Speaker 1: the authors point out some interesting things, one of which
Speaker 1: is that when blood osmolality and blood volume are both
Speaker 1: threatened at the same time, for example, if they both
Speaker 1: increase above the ideal range at the same time, the
Speaker 1: body places a higher priority on defending the ideal osmolality
Speaker 1: than it does on defending volume. So examples of this
Speaker 1: would be, you know, hypernetremia having too much sodium, or
Speaker 1: hyperglycemia having too much glucose in the blood, whatever it is.
Speaker 1: The state of having too high of an osmolality, which
Speaker 1: they call hyper tonicity, is probably more threatening, more of
Speaker 1: a danger overall than having not quite the right amount
Speaker 1: of water volume in your body. But anyway, well, you know,
Speaker 1: so what takes care of this whole job. Well, you've
Speaker 1: got some physical structures in your brain that sort of
Speaker 1: dip their finger into the soup to taste it for
Speaker 1: salt and let you know what's going on. Both of
Speaker 1: them are small, They reside in the forebrain, and they
Speaker 1: are known as first of all, the subfornical organ or SFO,
Speaker 1: and then the organum vasculosum of the lamina terminalis or OVLT.
Speaker 1: Now you've probably heard before of something called the blood
Speaker 1: brain barrier. This is a system of borders that prevent
Speaker 1: things that happen to be floating around in the blood
Speaker 1: from passing non selectively into the brain. So you know,
Speaker 1: the brain does need blood. It has to receive oxygen
Speaker 1: and other nutrients from blood flow, but the brain has
Speaker 1: to also protect itself against totally unregulated exchange with the blood.
Speaker 1: And there may be multiple reasons for this, but one
Speaker 1: of the main ones I've seen is that this prevents
Speaker 1: blood borne pathogens from infecting brain tissue, so that seems important.
Speaker 1: So in regions where it's active, the blood brain barrier
Speaker 1: only allows selective passage of certain types of material from
Speaker 1: the blood into the neurons. But if you're part of
Speaker 1: the brain that needs to get raw data about the
Speaker 1: contents of the blood moment by moment, apparently it won't
Speaker 1: do to be hiding behind this protective fence of cells. So,
Speaker 1: according to the authors here, the SFO and the OVLT,
Speaker 1: though they are in the brain, are located outside the
Speaker 1: blood brain carrier, so they can sort of taste the
Speaker 1: blood river unfiltered, undiluted to get a raw sense of
Speaker 1: what's going on. To read directly from the paper here quote,
Speaker 1: it is thought that these SFO and OVLT neurons monitor
Speaker 1: the blood osmolality directly, possibly via stretch sensitive ion channels
Speaker 1: embedded in their plasma membranes that detect changes in cell
Speaker 1: volume following intracellular dehydration. However, the identity of the specific
Speaker 1: ion channel or other protein responsible for OSMO sensing by
Speaker 1: these neurons is unknown. Furthermore, the possibility cannot be excluded
Speaker 1: that other cell types such as glia, play an important
Speaker 1: role in OSMO sensation. So here they identify, they put
Speaker 1: the flag up. For One more thing that hasn't fully
Speaker 1: been figured out in the science of thirst is what
Speaker 1: is the direct molecular mechanism that the neurons in these
Speaker 1: brain regions use to detect changes in blood osmolality. Maybe
Speaker 1: these stretched sensitive cells that you get dehydrated and then
Speaker 1: send information based on that to the brain regions that
Speaker 1: then filter out to other brain regions from there, But
Speaker 1: we don't know for sure anyway that is sensing intracellular
Speaker 1: dehydration or increases in blood osmolality. But what about that
Speaker 1: other kind extracellular dehydration where the blood volume actually decreases overall,
Speaker 1: such as after bleeding. Well. Decreases in blood volume are
Speaker 1: known as hypovolemia, like low volume hypovolemia, and they correspond
Speaker 1: with a decrease in blood pressure hypotension, and the body's
Speaker 1: reactions to hypotension take places sort of a complex chain
Speaker 1: of mechanisms involving multiple organs and several forms of a hormone,
Speaker 1: very important hormone called angiotensin. You might see this abbreviated
Speaker 1: sometimes with like capital letters A in GI and then
Speaker 1: maybe A and GII for angiotensin one and angiotensin two.
Speaker 1: Apparently the most crucial form of the hormone is angiotensin two,
Speaker 1: which causes things like the narrowing of blood vessels. So
Speaker 1: if you shrink blood vessels and make them smaller, that
Speaker 1: helps keep blood pressure up when volume is low, and
Speaker 1: it also leads to water reuptake by the kidneys. The
Speaker 1: kidneys are like holding fast to the water content rather
Speaker 1: than just squandering it as they might if you had
Speaker 1: plenty of water in your body. And there's evidence that
Speaker 1: the presence of angiotensin two this hormone also causes a
Speaker 1: thirst drive to be generated in the brain, primarily involving
Speaker 1: the SFO or the subforonical organ Interestingly, in rodents, if
Speaker 1: you just give them straight angiotensin two, it causes what
Speaker 1: the authors call profound water consumption, just like voluminous drinking
Speaker 1: of water. But in humans, apparently, angiotensin two levels quote
Speaker 1: do not correlate well with the perception of thirst, and
Speaker 1: infusions of physiologic levels of angiotensin two do not stimulate drinking. Interesting, so,
Speaker 1: perhaps while this hormone does things to increase blood pressure,
Speaker 1: like constrict blood vessels and cause water reuptake by the kidneys,
Speaker 1: it does not make us thirsty for water like it
Speaker 1: does in other animals for some reason, the authors write quote.
Speaker 1: While this suggests that angiotensin two might be less important
Speaker 1: for regulation of drinking in humans, interpretation of these negative
Speaker 1: results is complicated by the fact that peripheral infusion of
Speaker 1: angiotensin two rapidly increases blood pressure, which can then feed
Speaker 1: back to counteract any effects of angiotensin two on thirst.
Speaker 1: So maybe they're just complicated balancing interactions here that when
Speaker 1: you know if you directly infuse this hormone, it increases
Speaker 1: blood pressure, which has other downstream effects which which counteract
Speaker 1: the onset of thirst. Okay, I'm I guess just it
Speaker 1: just goes to show, you know, one of the factors
Speaker 1: of experiments with with with mice and other non human
Speaker 1: mammals is that we have a whole lot in common
Speaker 1: with them, but not everything is going to it's going
Speaker 1: to apply to us one to one, right, A lot does,
Speaker 1: but not everything. Okay, But anyway, I guess what we're
Speaker 1: trying to figure out here is building a bridge from
Speaker 1: our existing knowledge about these brain regions that play a
Speaker 1: role in monitoring the body's osmolality and water content and
Speaker 1: u and regulate in these hormones that help regulate the
Speaker 1: body's physiologic response to dehydration, and how that connects to
Speaker 1: the actual behavior and the drives that we sense when
Speaker 1: we get thirsty and go get water. Um so there
Speaker 1: there are sort of system wide homeostatic responses when when
Speaker 1: we get dehydrated again. The SFO and the OVLT, together
Speaker 1: with another region in between them, the median preoptic nucleus
Speaker 1: or MNPO. These three regions together comprise a hub called
Speaker 1: the lamina terminalys. This is sort of the brain's fluid
Speaker 1: control center. What is fairly well understood is the autonomic
Speaker 1: and neuroendocrine pathways by which the body responds to dehydration.
Speaker 1: You know all these things we've been talking about. The
Speaker 1: angiotensin too, the constriction of blood vessels to increase blood pressure,
Speaker 1: water reuptake by the kidneys, the release of other hormones,
Speaker 1: not just angiotensin, but things like vasopressin and oxytocin. But
Speaker 1: what is less well understood are the mechanisms leading to
Speaker 1: the generation of thirst as a motivation state. But we
Speaker 1: know some things that are very interesting. So to come
Speaker 1: back to something you actually mentioned earlier in the episode, Rob,
Speaker 1: the brain's regulation of water drinking is not based only
Speaker 1: on the current osmolality of the blood. So it's not
Speaker 1: just these brain regions that you dip a finger in
Speaker 1: and see how salty the soup is. The rain also
Speaker 1: appears to change our motivation to drink water before changes
Speaker 1: actually show up in the blood. Their behavior changes that
Speaker 1: occur in anticipation of changes in osmolality. So what would
Speaker 1: this mean in plain English? Okay, so you're out in
Speaker 1: the hot sun, wrestling alligators, or you know, you're working
Speaker 1: up a sweat, whatever it is you're doing, and you
Speaker 1: come in, you get in the shade, and you drink
Speaker 1: a nice glass of water. When you do that, there
Speaker 1: is actually a delay on the order of tens of minutes.
Speaker 1: It might be ten, twenty or even more minutes before
Speaker 1: the water that you just drank is fully absorbed by
Speaker 1: the digestive system and added into the blood. However, it
Speaker 1: might just take you a few seconds to drink a
Speaker 1: glass of water and then decide whether you're going to
Speaker 1: drink more. So, if it's taking your body tens of
Speaker 1: minutes to fully incorporate the water you've just consumed and
Speaker 1: for that to show up in a blood o molality test,
Speaker 1: how come you don't just keep drinking water constantly until
Speaker 1: that happens. You know, you don't. Even if you're thirsty,
Speaker 1: you don't usually drink a glass of water and then
Speaker 1: just fill up another one, and another and another and
Speaker 1: another for you know, fifteen minutes or something. If your
Speaker 1: level of thirst were only based on your blood osmlality,
Speaker 1: you might do that. You might kill yourself just drinking
Speaker 1: gallons of water while you're waiting for your fluid monitoring
Speaker 1: system to register the changes. Right, Yeah, yeah, I mean,
Speaker 1: I mean it needs to be Again, it comes back
Speaker 1: to just what a fine balance it is, and therefore
Speaker 1: you need you need different sensors in different places in
Speaker 1: the human mechanism here, right, So the sensation of your
Speaker 1: thirst being quenched by drinking water must be created by
Speaker 1: a different process altogether the author's right quote. Thirst is
Speaker 1: not quenched by the reverse of the process that generates it. Instead,
Speaker 1: the brain appears to somehow detect the intake of liquid.
Speaker 1: It's usually thought that this happens somewhere in the oropharyans
Speaker 1: in the middle part of the throat, and then somehow
Speaker 1: adjusts the feeling of thirst in anticipation of the coming
Speaker 1: changes in blood osmolality. So it looks like what happens
Speaker 1: is that you're dehydrated, you drink water. The swallowing of
Speaker 1: water is somehow sensed in the throat, and then these
Speaker 1: sensations are transmitted to the subformical organ, the sfo, and
Speaker 1: then from there they inhibit thirst generating pathways. And so
Speaker 1: this raises interesting questions how exactly do we sense water
Speaker 1: intake in the throat? This was not well understood at
Speaker 1: the time this paper was written. I actually have come
Speaker 1: across a couple of studies in the years since that
Speaker 1: we can talk about as we go on. We might
Speaker 1: get more into those in the next part of the series.
Speaker 1: But at the time, some of the ideas out there were, well,
Speaker 1: maybe it has something to do with temperature. Apparently cooling
Speaker 1: of the throat triggers water intake signals, and evidence for
Speaker 1: this claim would be that research has found that cold
Speaker 1: liquids inhibit thirst faster than warm liquids. I haven't tried
Speaker 1: it myself, but that sounds correct to me. Interesting and
Speaker 1: it would make sense water usually tends to cool the
Speaker 1: mouth in the throat, and that this cooling may be
Speaker 1: used as a rough signal that water is coming in. Also,
Speaker 1: it seems that cooling of the mouth on its own,
Speaker 1: even if it's not water, just making the mouth colder,
Speaker 1: can somehow reduce thirst and reduce activity of the SFO. Interesting,
Speaker 1: so you would potentially be able to use just like
Speaker 1: a cooling breath of technique to inhibit thirst, possibly if
Speaker 1: you know, if the temperature explanation has anything going for it.
Speaker 1: But there may be other things as well. There may
Speaker 1: be other ways of sensing water in the oral cavity.
Speaker 1: Maybe some stuff more along the lines of what you
Speaker 1: were talking about earlier. Things could have to do with
Speaker 1: taste sensors that somehow to detect the presence of water
Speaker 1: through acid sensing taste receptors or something, or there could
Speaker 1: be there may be some limited evidence that stretch receptors
Speaker 1: and osma sensors in the stomach might also detect water
Speaker 1: intake before full absorption. But again, at the time of
Speaker 1: this summary in twenty sixteen, this was just not fully understood.
Speaker 1: I guess one way of looking at it would be
Speaker 1: that it's it's not necessarily like one trigger or one
Speaker 1: sensor that is that's playing a crucial role here. It's
Speaker 1: more of a whole suite of things that is that
Speaker 1: is generating this understanding of how much water has entered
Speaker 1: the system. Right. But whatever the exact mechanism is, it's
Speaker 1: definitely anticipatory in nature. It's definitely changing your behavior before
Speaker 1: the thing that your behavior is supposed to fix has
Speaker 1: actually changed. Yet. Wow, Like, when you're done drinking the
Speaker 1: glass of water, you're still dehydrated. You're going to be
Speaker 1: dehydrated for another ten to fifteen minutes at least. It's
Speaker 1: it's the kind of excellence in supply chain management that
Speaker 1: technology companies are chasing after, you know, the idea that
Speaker 1: they want to anticipate the need um and then you know,
Speaker 1: be able to h to alter the supply chain, you know,
Speaker 1: at moment to moment to make sure that the need
Speaker 1: is met without wasting water or product or whatever the
Speaker 1: case may be. Yeah, I like it. Okay, there's more
Speaker 1: anticipation to come. How about anticipatory regulation of thirst from
Speaker 1: eating from food? Because when we eat food, thirst appears
Speaker 1: to be generated in anticipation of coming changes to blood osmolality.
Speaker 1: So why would eating food make us need water? Well,
Speaker 1: first of all, fluids are used in digestion. You think
Speaker 1: about when you eat food, you generate saliva you uh,
Speaker 1: you know, not to be gross, but there's a lot
Speaker 1: of lubrication that needs to happen, Like swallowing requires some water. Oh,
Speaker 1: I mean you have to you have to chew up
Speaker 1: the food and form the bolus that will then travel
Speaker 1: down the throat. I mean, I think we've we've covered
Speaker 1: that on the show before. Like, think about it today
Speaker 1: your next meal. Really focus on everything that's going on
Speaker 1: in your mouth. It's a it's a beautiful chorus of
Speaker 1: disassembly and then reassembly into the right sort of package
Speaker 1: to then make the journey down to the stomach. It's
Speaker 1: really it's like, it's beautiful and it's horrible. You can't
Speaker 1: think about it while you're eating, or at least I
Speaker 1: find I can't just think. I mean, have you ever
Speaker 1: thought of a more appetizing idea that you're like, mouth
Speaker 1: and throat are lubricating the ball of food that you're
Speaker 1: smashing up. Yeah, but anyway, so that's one thing. But
Speaker 1: then the other half of it is that eating usually
Speaker 1: increases blood os molality by adding salt and other things
Speaker 1: to the body. Yes, yes, so much salt. So much
Speaker 1: so the body appears to have an anticipatory response to
Speaker 1: eating that is generated eating thirst before those changes even register.
Speaker 1: And this thirst that comes from eating is known as
Speaker 1: pran deal thirst. It seems to occur before changes in
Speaker 1: blood osmolality come into effect, so a lot of animals
Speaker 1: are observed to drink at the same time that they eat,
Speaker 1: if they're able. The mechanism of this anticipatory neural pathway
Speaker 1: is still not fully understood, but if prandial thirst is
Speaker 1: not sated. I mentioned this earlier. Sometimes animal brains tend
Speaker 1: to react by reducing appetite until water is consumed. This
Speaker 1: is known as dehydration induced stanorexia. But in general, dehydration
Speaker 1: will cause animals to restrict their food intake, with some exceptions,
Speaker 1: of course, because some animals, you know, they get their
Speaker 1: water entirely from food and so forth. But dehydration induce
Speaker 1: stanorexia tends to I thought this was interesting. Reduce meal
Speaker 1: size but not meal number, so it might not affect
Speaker 1: how often an animal is willing to eat, but how
Speaker 1: much they eat when they do eat, meaning that it
Speaker 1: probably works by causing an animal to terminate feeding behaviors
Speaker 1: earlier than they would normally each time it has a meal.
Speaker 1: Now at the end of their rite. Up Here, the
Speaker 1: authors acknowledge that acknowledge some exceptions to the stuff they're
Speaker 1: talking about. These generalizations tend to be true for humans
Speaker 1: and rodents and some other animals, but of course they're
Speaker 1: very different ecological niches that will cause variation to these generalizations.
Speaker 1: For example, a lot of grazing herbivores do not seem
Speaker 1: to experience pran deal thirst or thirst related to eating.
Speaker 1: You know, because they eat all day, but they only
Speaker 1: drink drink water a few times a day. And then
Speaker 1: they're very different kinds of animals like amphibians that don't
Speaker 1: technically drink at all, like many amphibians just absorb water
Speaker 1: through their skin. We've talked about amphibians before in the
Speaker 1: show and about how the you know, how delicate their
Speaker 1: place in the environment can be, and this is this
Speaker 1: is part of it. It's like if if if I'm
Speaker 1: in the environment, i am drinking it, I am breathing it,
Speaker 1: and my skin is the is the barrier through which
Speaker 1: all this takes place. Yeah, very different relationship to one's environment,
Speaker 1: a very very different kind of chemistry of being for
Speaker 1: the for the amphibian, I recall it's been a while,
Speaker 1: but I recall reading a funny article about terrestrial toads
Speaker 1: that would primarily do water absorption through their pelvic regions,
Speaker 1: so they sort of like like thrust their lower bodies
Speaker 1: and bellies into any surface that's wet while they're while
Speaker 1: they're hanging out in water absorption mode. All right, We're
Speaker 1: gonna ahead and close out this episode, but we will
Speaker 1: be back for a third thirst episode. We have a
Speaker 1: lot of additional information we want to get to. Don't
Speaker 1: worry there will be parasites in it, So it should
Speaker 1: be it should be a grotesque a good time. And
Speaker 1: of course, in the meantime, right in, let us know
Speaker 1: your thoughts on water. All of you are water drinkers.
Speaker 1: You consume water one way or another or another, and
Speaker 1: I imagine you have some thoughts on all of this.
Speaker 1: Do you do you love drinking water? Do you hate
Speaker 1: drinking water? Is there? What are your thoughts on the
Speaker 1: consumption of cold water versus hot water or maybe prefer
Speaker 1: the lukewarm water? I mean, I guess they're factors we
Speaker 1: didn't even get into, you know, like sometimes cold water
Speaker 1: can be sensitive on the teeth and gums. The same
Speaker 1: can be said for hot water as well. So I
Speaker 1: don't know. Perhaps you have thoughts on that. Have you
Speaker 1: never drunk water? You? Are you a creature that only
Speaker 1: absorbs water from chicken nuggets? Oh the chicken chicken nugget water? Yes? Well, anyway,
Speaker 1: right and let us know it's all fair game. In
Speaker 1: the meantime as usual, Core episodes of Stuff to Blow
Speaker 1: Your Mind Tuesdays and Thursdays and the Stuff to Blow
Speaker 1: Your Mind Podcast feed. We have a listener meant on Monday,
Speaker 1: Artifact on Wednesday, and on Friday we do Weird House Cinema.
Speaker 1: That's our time. Just had most serious matters aside and
Speaker 1: just talk about a strange film. Huge thanks, as always
Speaker 1: to our excellent audio producer Seth Nicholas Johnson. If you
Speaker 1: would like to get in touch with us with feedback
Speaker 1: on this episode or any other, to suggest a topic
Speaker 1: for the future, or just to say hello, you can
Speaker 1: email us at contact at stuff to Blow your Mind
Speaker 1: dot com. Stuff to Blow Your Mind is production of iHeartRadio.
Speaker 1: For more podcasts from my Heart Radio, visit the iHeartRadio app,
Speaker 1: Apple Podcasts, or wherever you're listening to your favorite shows.